1. Field of the Invention
This invention relates to a process for the drying of water-wet solids and sludges of various kinds, for solvent-extraction of indigenous solvent-soluble compounds from said solids and sludges, and for recovering solids, water, and indigenous solvent-soluble compounds for further use. More particularly, it deals with a continuous process for drying and solvent-extraction of solids and sludges wherein the input material is mixed with water-immiscible solvents of various kinds to obtain a mixture that remains fluid and pumpable even after virtually all of the water has been removed, and to extract indigenous solvent-soluble compounds from the input material.
2. Discussion of the Prior Art
Drying and solvent-extraction of water-wet solids and sludges (hereinafter referred to as "waste solids") is the object of large and varied industries. Examples of waste solids requiring such treatment include, but are not limited to:
(1) Municipal and industrial sewage sludges, such as raw primary sludges, waste activated sludges, anaerobically digested sludges, and biosludges; PA0 (2) Animal wastes, such as pig manures, wool-scouring wastes, chicken manures, and cow manures; PA0 (3) Contaminated soils, such as soils contaminated with crude oils, fuel oils, polychlorinated biphenyls, polynuclear aromatics, coal tars, and oil drilling muds; PA0 (4) Refinery sludges, such as API separator sludges, dissolved air flotation floats, and slop oil emulsion solids; PA0 (5) Ink and dye sludges; PA0 (6) Alum sludges; PA0 (7) Wood pulp mill activated sludges and black liquirs; PA0 (8) Pharmaceutical plant wastes; PA0 (9) Brewery sludges; PA0 (10) Dairy and food products and wastes, such as milk whey by-products, coffee wastes, and chocolate wastes; PA0 (11) Peats, lignites, and brown coals; and PA0 (12) Meat rendering wastes.
Drying and solvent-extraction of waste solids present many processing problems relative to the efficiency and reliability of production. Various typical processes for dehydrating waste solids using solvent extraction technologies are disclosed in U.S. Pat. Nos. Re. 26,317; Re. 26,352; 3,323,575; 3,716,458; 3,855,079; 3,950,230; 4,013,516; 4,270,974; 4,418,458; 4,336,101; and 4,702,798.
In general, the processes and apparatus described in the aforementioned patents involve slurrying waste solids, such as one or a combination of the types listed above, with a water-immiscible solvent to obtain a mixture which remains fluid and pumpable even after virtually all of the water has been removed. The properties of the solvent can be varied over a wide range to achieve the desired characteristics. The solvent should be immiscible in water and should have an atmospheric boiling point of 300.degree. F. or higher to prevent excessive evaporation of the solvent during the evaporation of water from the solvent. The viscosity of the solvent should be low enough, typically less than 500 cp, so that the slurry is pumpable at the flowing temperatures. Extraction of compounds from the input waste solids can be enhanced by changing the chemical composition of the solvent to increase the solubility of the compounds in the solvent. The chemical composition of the solvent can also be adjusted to improve the dispersibility of the waste solids in the solvent. Isopar "L" and Amsco 140 are the trade names for solvents which meet the above criteria and have been used in these processes. Isooctanol is an example of another solvent which has been used in these processes.
The resulting mixture of solvent and waste solids is passed through a sequence of drying steps in which the mixture is dried by heat evaporation, becoming increasingly dry during each subsequent step. Economies of energy consumption are realized by utilizing the evolved vapor from each evaporation step to supply a substantial portion of the heat requirements of another evaporation step. The evaporation steps generate a slurry of dried or partially-dried solids in solvent which is withdrawn and fed to a centrifuge (or other apparatus for separting liquids from solids) to separate a substantial portion of the solvent from the solids. The solids leaving the centrifuge are sometimes processed further by heating them in a "desolventizer", referred to as a cake deoiler and specifically disclosed in U.S. Pat. No. 4,270,974. In the desolventizer, blowing steam, purge gas, and/or vacuum are used to recover virtually all of the remaining solvent from the solids. In many cases, the centrifuge centrate is fed to a distillation system where the indigenous solvent-soluble compounds extracted from the solids are separated from the solvent and recovered for final disposition.
It has been found that, when solids containing non-adsorbed (or "free") water are suspended in solvent, they may become "sticky" and form relatively large masses that adhere to the inner walls of the pipelines, heat exchanger tubes, and vessels. There are also cases where circulation is prevented due to the sticky solids settling out and plugging the pipelines or circulating pumps. The sticky solids may coat the heat exchanger surfaces, reducing the heat transfer rate in the heat exchangers so that the water evaporation rate is reduced markedly and the capacity of the evaporation system becomes low. Further, when the sticky solids adhere to the walls of the pipelines, heat exchangers, or vessels, they may cause corrosion if the solids are acidic in nature or contain other corrosive compounds. There are also cases where sticky solids in mixing tanks have a tendency to settle out and disproportionate amounts of water-wet solids in solvent are pumped to the evaporation stages, causing upsets in the stability of the system. Improvements have been developed in the processes and apparatus to overcome the problems of sticky solids. In one case, described in U.S. Pat. No. Re. 31,185, a portion of the slurry containing dried or partially-dried solids and solvent from one of the evaporation steps is recycled and mixed with the input material to reduce the average moisture content (thereby eliminating the "free" water present) of the material entering the first evaporation stage. In another case, described in U.S. Pat. No. 4,702,798, a surface active agent is mixed with the solvent and input material to prevent the formation of sticky solids and to keep the solids well-dispersed in the solvent. While successful in overcoming the problems of sticky solids, each of these improvements involves substantial increases in the cost of processing, either through higher flow rates, larger and more complex process equipment, and/or the added cost for surface active agents.